It is well known that radio waves, propagating from a transmitter to a receiver, can follow a plurality of different paths, and that the relative phases of the different waves arriving at the receiving antenna can be such as to destructively interfere, causing what is commonly referred to as a fade. In order to reduce the opportunity for this to occur, the so-called "space diversity" system has been developed using two, spaced antennas to feed a common or two separate receivers. The theory underlying the use of two spaced-apart antennas is that there is less likelihood that a fade will occur at both antennas at the same time. In the simplest system using a single receiver, means are provided to disconnect the receiver from one antenna as soon as the received signal level falls below a predetermined threshold and to connect the receiver to the second antenna. In this so-called "blind switching", it is assumed that the signal received by the second antenna is stronger than that received by the first antenna. In a more sophisticated system, the signals from the two antennas are combined instead of switching between them. This eliminates amplitude and phase jumps associated with the switching operation, and has the added advantage of delivering a larger amplitude signal to the receiver. However, such a system requires the use of dynamic phase correction to compensate for variations in the relative phase of the two signals caused by changes in the path lengths traversed by them. (See, for example, U.S. Pat. No. 4,160,952.)
A second difficulty resides in the manner in which the phase shifter control signal is derived. Typically, a small phase modulation is impressed upon the signal in one of the two antenna circuits. The phase modulation produces an amplitude modulation of the composite signal obtained when the two signals are combined. The fundamental second harmonic of this amplitude modulation is then detected by the receiver's AGC circuit and used to control the phase shifter. The problem with this approach is that it is often difficult to accurately detect the relatively small second harmonic component in the presence of noise.
In wideband radio systems, it has been found that amplitude and delay distortion from multipath fading can, under certain conditions, be a major cause of transmission deterioration. Means for detecting and compensating for this effect are disclosed in the copending application of H. Miedema, Ser. No. 158,404, filed June 11, 1980, and assigned to applicant's assignee. The process, which includes in-band equalization, can be involved and expensive.